1. Field of the Invention
The invention generally relates to the field of optical telecommunications, and more particularly to the monitoring of phase-shift keyed (PSK) optical signals.
2. Description of the Related Art
Optical telecommunications rely on data-encoding of information in the amplitude and/or phase of light from an optical source. Various methods for encoding information are known in the art. For example, analog modulation directly encodes the information as a continuous variable on the source of light. Digital communication encodes a digitized version of the information where discrete levels of a variable in the encoded source, for example the amplitude of the pulses, correspond to bits of information. Regardless of the choices made for the encoding, noise from various sources can potentially deteriorate the quality of the information encoded on the channel, and can lead to errors in the transmission of information. Therefore, being able to optimize a transmitter for optimal performance and quantifying the apparition of noise on a channel during transmission are crucial tasks when operating an optical telecommunication network.
Various optical monitoring techniques are known and, for example, are used to track the amount of amplified spontaneous emission present on a channel. Most impairments modify both the temporal intensity and phase of the electric field of an optical channel. While such temporal intensity is modulated by the data encoding process in an amplitude shift keyed (ASK) signal, the intensity is expected to be a periodic function of time (with a periodicity equal to the bit period) in the case of a phase shift keyed (PSK) signal. Therefore, any non-periodic behavior of the temporal intensity is a sign of impairment in a PSK channel. Such a property and various derivations can be put in practice to monitor phase-shift keyed signals.